Lithographic and gravure printing techniques have been refined and improved for many years. The basic principle of lithography is transferring ink from a surface having both ink-receptive and ink-repellent areas. Offset printing incorporates an intermediate transfer of the ink. For example, an offset lithographic press may transfer ink from a plate cylinder to a rubber blanket cylinder, and then the blanket cylinder transfers the image to the web (i.e., paper). In gravure printing, a cylinder with engraved ink wells makes contact with a web of paper and an electric charge helps transfer the ink onto the paper.
Early implementations of lithographic technology utilized reliefs of the image to be printed on the plate such that ink would only be received by the raised areas. Modern lithographic processes take advantage of materials science principles. For example, the image to be printed may be etched onto a hydrophilic plate such that the plate is hydrophobic in the areas to be printed. The plate is wetted before inking such that oil-based ink is only received by the hydrophobic regions of the plate (i.e., the regions of the plate that were not wetted by the dampening process).
However, all of these printing techniques have a similar limitation. The same image is printed over and over again. Lithographic printing uses plates containing a permanent image, whether it be a relief image or an etched hydrophobic image, etc. Gravure printing also uses a permanent image which is engraved in ink wells on a cylinder. Therefore, lithographic and gravure presses have not been used for printing “short-run” jobs or jobs containing variable data (e.g., billing statements, financial statements, targeted advertisements, etc.). There is a substantial overhead cost involved in making the plates that are used by a lithographic press. Therefore, it is not cost effective to print a job on a lithographic press that will have few copies produced (i.e., a short-run job). Furthermore, the content cannot be varied, such as in laser printing and ink jet printing.
Traditionally, many printed articles such as books and magazines have been printed using a process that involves a great deal of post-press processing. For example, a single page of the magazine may be printed 5,000 times. Then, a second page may be printed 5,000 times. This process is repeated for each page of the magazine until all pages have been printed. Then, the pages are sent to post-processing for cutting and assembly into the final articles. If variable images could be printed at lithographic image quality and speed, each magazine could be printed in sequential page order such that completed magazines would come directly off the press. This would drastically increase the speed and reduce the expenses of printing a magazine.
Ink jet printing technology provided printers with variable capability. There are two main ink jet technologies: bubble jet (i.e., thermal) and piezoelectric. In each, tiny droplets of ink are fired onto a page. In a bubble jet printer, a heat source vaporizes ink to create a bubble. The expanding bubble causes a droplet to form, and the droplet is ejected from the print head. Piezoelectric technology uses a piezo crystal located at the back of each ink reservoir. Electric charges are used to cause vibrations in the crystals. The back and forth motion of the crystal is able to draw in enough ink for one droplet and eject that ink onto the paper.
The quality of color ink jet printing is generally orders of magnitude lower than that of offset lithography and gravure. Furthermore, the speed of the fastest ink jet printer is typically much slower than a lithographic or gravure press. Traditional ink jet printing is also plagued by the effect of placing a water-based ink on paper. Using a water-based ink may saturate the paper and may lead to wrinkling and cockling of the print web. In order to control these phenomena, ink jet printers use certain specialized papers or coatings. These papers can often be much more expensive than a traditional web.
Furthermore, when ink jet technology is used for color printing, the ink coverage and water saturation is increased. This is due to the four color process that is used to generate color images. Four color processing involves laying cyan, magenta, yellow and black (i.e., CMYK) ink in varying amounts to make any color on the page. Thus, some portions of the page may have as many as four layers of ink if all four colors are necessary to produce the desired color. Additionally, the dots produced by an ink jet printer may spread and produce a fuzzy image.
Laser printing does not appear to be a viable alternative for high speed variable printing at present, because production speeds are still much slower than offset and gravure, and the material costs (e.g., toner, etc.) are extremely high. Laser color is also difficult to use for magazines and other bound publications, because the printed pages often crack when they are folded.
Therefore, it would be desirable to develop a variable printing technique having the quality and speed of traditional lithographic and gravure printing. It would further be desirable to provide a variable printing system that operated at speeds of at least 400 feet per minute.